Improved high-performance liquid chromatographic assay for theophylline in plasma and saliva in the presence of caffeine and its metabolites and comparisons with three other assays

A new ion-pairing reversed-phase high-performance liquid chromatographic assay for theophylline is described which allows the separation of theophylline from 1,7-dimethylxanthine — a metabolite of caffeine which interferes with most theophylline assay procedures. Levels of 1,7-dimethylxanthine equivalent to 3 mg/l theophylline were seen in individuals not taking theophylline but who drank three to four cups of coffee per day. This compound was not seen in individuals abstaining from xanthine-containing foods and beverages.